A beam splitter is an optical component which splits a single beam into two partial beams. If a beam is incident on a beam splitter, then a portion of the incident beam is reflected and the other portion is transmitted. In the case of a dichroic beam splitter, depending on the wavelength of the incident beam, different transmission values or reflection values are obtained. Scattering losses are very small in the case of dichroic beam splitters, so that the degree of transmission plus the degree of reflection is nearly one.
Dichroic beam splitters are used for example in fluorescence meters, where they are usually configured as a dichroic beam splitter plate. Dichroic beam splitter plates have a dichroic coating on their surface. The surface normal of these plates is generally offset by 45° from the beam path. In the case of dichroic beam splitter plates the transmission characteristic over wavelength (transmission is a function of wavelength) includes a relatively steep transition edge of no to high transmission values, or vice versa.
As a result, dichroic beam splitter plates can find good use in the excitation of fluorescent materials, in which the excitation wavelength and the emission wavelength are relatively close together.
EP 1 856 509 B1 describes a fluorescence meter for examining a sample, comprising a main beam path and at least one optical module. The optical module is embodied so as to provide at least one electromagnetic beam for exciting the sample and receive at least one electromagnetic beam emitted by the sample. The fluorescence meter comprises at least one source for providing electromagnetic beams, which is monitored by means of a monitor diode. Preferably, several beam splitters which are configured as dichroic mirrors (dichroic beam splitter plates) are arranged in the main beam path. The dichroic mirrors or dichroic beam splitter plates can be regarded as high-pass filters which transmit longer wavelength electromagnetic beams and reflect shorter wavelength beams. They are optimized for an angle of incidence of the electromagnetic beam of 45°. By means of dichroic mirrors, for example, exciting electromagnetic beams of the source are coupled into the main beam path and focused on the sample.
From the prior art, beam splitter cubes are also known, which consist of two interconnected prisms, between which a dielectric layer is arranged. In EP 2 711 762 A1, a non-polarizing beam splitter cube is described, which has a sequence of layers of refractive layers of dielectric material with different refractive indices.
Non-polarizing beam splitter plates, as well as non-polarizing beam splitter cubes, split the incident light in a particular transmission/reflection ratio while maintaining the original polarization state. A dichroic function is not present here. All wavelengths are split in a particular ratio, e.g., 50% to 50%.
U.S. Pat. No. 5,400,179 A is concerned with a beam splitter consisting of two prisms, between which an optical layer of six sublayers is arranged. Adjacent sublayers have a different refractive index. The angle of incidence of a beam impinging on the optical layer is at least 40°. This publication does not address dichroic beam splitters. Rather, non-polarizing beam splitters are addressed. Such beam splitters can not be used, e.g., for fluorescence measurements since they do not have a transmission characteristic over the wavelength (transmission is a function of the wavelength) which has a transition edge of no to high transmission values, or vice versa.
JP 2009-31406 A shows a non-polarized beam splitter consisting of two prisms and a dielectric layer arranged between the two prisms consisting of several interconnected sublayers. The sublayers have different refractive indices. While at least one sublayer has a higher refractive index than the prisms, at least one other sublayer has a lower refractive index than the prisms. The beam splitter can consist of two prisms with a triangular or trapezoidal base surface. In the embodiments having a trapezoidal base surface, the angle of incidence of a beam impinging on the dielectric layer is substantially greater than 45°. In the exemplary embodiment described, the angle of incidence is 72°.
A beam splitter consisting of two trapezoidal prisms is known from WO 2010/025536 A1. Between the prisms there is a coating structure with an upper layer, a spacer layer and optionally a lower layer. The spacer layer encloses a cavity filled with non-reactive gas or vacuum which acts as an interference layer. The coating structure allows for thin film interference in conjunction with frustrated total reflection within the cavity for predetermined angles of incidence.
U.S. 2013/0308198 A1 discloses a dichroic beam splitter consisting of at least two interconnected prisms. The beam splitter has three outer surfaces arranged in different planes. Disposed in the beam splitter is a dichroic layer which crosses at least one of the surfaces.
Dichroic beam splitter cubes are also known from the prior art. Dichroic beam splitter cubes also split incident light into certain wavelength ranges. In the case of dichroic beam splitter cubes, too, depending on the wavelength of the incident beam, different transmission values or reflection values are obtained. Scatter losses are also very small in the case of dichroic beam splitter cubes, so that the degree of transmission plus the degree of reflection is nearly one. In the case of dichroic beam splitter cubes the transmission characteristic over the wavelength (transmission is a function of the wavelength) includes a relatively shallow transition edge from no to high transmission values, or vice versa. They are therefore not suitable for measurements in which the excitation wavelength and emission wavelength are relatively close together. This is the case, for example, in the measurement of fluorescent light, since for many fluorescent dyes the excitation wavelength and the emission wavelength are very close together. Typically, the excitation wavelength and the emission wavelength are separated by 20 to 30 nm, although a smaller separation is also possible. If the slope of the transmission characteristic is not steep enough, the transition range from high reflection to high transmission will extend clearly over a larger wavelength range than the 20 to 30 nm mentioned above. The excitation light would therefore falsify the measuring result. An advantage of beam splitter cubes is that further optical components can be arranged directly on the side surfaces of the beam splitter cube. This reduces the installation effort because the optical components can already be positioned correspondingly on the beam splitter cube.
An arrangement for the representation of micro-array data is described, for example, in U.S. 2014/0206580 A1. The arrangement comprises, inter alia, a dichroic mirror or a beam splitter. The beam splitter consists of two symmetrical, pentagonal beam splitter components. The beam splitter can be arranged with a central axis offset to the longitudinal axis of a support rod, on which the beam splitter is attached. The angle between the central axis of the beam splitter and the longitudinal axis of the support rod should be 30° to 45°. The angle between the entry surface for a laser radiation and the contact surface of the two beam splitter components is at a constant 45°.
A disadvantage of the previously known measuring arrangements, which are used in particular for the measurement of fluorescence, is that they have long reached the limits of miniaturizability and require a high adjustment effort. Dichroic beam splitter plates have above all the disadvantage that adjoining optical components must be adjusted freely in space and very precisely to these beam splitter plates. In the closer vicinity of beam splitters, however, often other optical elements are needed, such as light-absorbing arrangements. One possibility for light absorbing arrangements is to interleave interfaces between air and dark solid structures. Such light-absorbing arrangements can not be directly connected to beam splitter plates and are only insufficiently miniaturizable.